1. Field of the Invention
This invention relates to a magnetoinductive flow measuring method for measuring the flow of a medium moving through a measuring tube that is equipped with two measuring electrodes positioned along a connecting line extending in an essentially perpendicular direction relative to the axis of the measuring tube, whereby a magnetic field is generated that extends at least in an essentially perpendicular direction relative to the axis of the measuring tube and to the connecting line of the measuring electrodes and whereby the flow rate of the moving medium through the measuring tube is determined by measuring the voltage, respectively collected at one or both measuring electrodes, in comparison with a reference voltage potential.
2. Description of the Prior Art
Magnetoinductive flow measuring processes of the type mentioned above have been in the public domain for some time and have been widely employed for a variety of applications. The fundamental principle of a magnetoinductive flowmeter for moving media goes all the way back to Faraday who as early as 1832 proposed the use of the electrodynamic induction principle for measuring flow rates. Faraday""s law of induction stipulates that in a medium flowing through a magnetic field and containing charge carriers, an electric field intensity is generated perpendicular to the flow direction and to the magnetic field. A magnetoinductive flowmeter utilizes Faraday""s law of induction by means of a magnet that typically consists of two magnetic poles, each with a field coil, and generates a magnetic field perpendicular to the direction of the flow in the measuring tube. Within that magnetic field each volume element of the flowing medium, traveling through the magnetic field and containing a certain number of charge carriers, contributes the field intensity generated in the volume element concerned to the measuring voltage that can be collected via measuring electrodes. In conventional magnetoinductive flowmeters, the measuring electrodes are designed either for conductive or for capacitive coupling with the flowing medium. One salient feature of magnetoinductive flowmeters is the proportionality between the measured voltage and the flow rate of the medium as averaged across the diameter of the measuring tube, i.e. between the measuring voltage and the flow volume.
In applied flowmetering operations, the magnetoinductive flow measuring process usually involves periodic directional alternation of the magnetic field. The prior art shows a variety of approaches to that effect. For example, magnetoinductive flow measurement can be accomplished using an alternating field in which case the field coils of the magnet typically receive a sinusoidal 50 Hz voltage directly from an AC line source. However, the measuring voltage generated by the flow between the measuring electrodes tends to be heterodyned by transformation noise as well as line voltage interference.
Current magnetoinductive flow measuring practice, therefore, generally employs a switched direct-current field. A switched continuous field of that nature is obtained by feeding a periodically polarity-alternating square-wave current to the field coils of the magnet. Also possible, however, is a magnetoinductive flow measurement process using a pulsating continuous field obtained by only periodically supplying the field coils of the magnet with a time-controlled square-wave current of unchanging polarity. Yet a method that periodically reverses the field current is preferred because alternating the polarity of the magnetic field permits the suppression of interference potentials such as galvanic noise.
Using a pole-reversible, switched constant-current field makes it necessary after each reversal to wait for the magnetic field to stabilize. That is followed by the up-slope integration of the measured voltage, for instance the voltage differential between the electrodes, until the field current polarity is again reversed. Waiting for the magnetic field to stabilize is important for achieving good measuring accuracy. As indicated in EP 0 809 089 A2, the measuring signal itself can be used during the transient phase of the magnetic field. That is not easily accomplished and, besides, the measuring signal is weaker during the transient phase than in the stabilized state given that the measuring signal is inherently proportional to the magnetic field.
Particular problems are encountered in magnetoinductive flow measuring especially when relatively high-speed decanting or racking processes are to be metered. Such a process essentially consists of three stages, i.e. the initial draw-off characterized by an accelerating flow pattern, followed by a constant flow rate, and finally the draw-off end stage characterized by a decelerating flow rate typically slowing to zero. In practice, the first i.e. starting stage in high-speed draw-off processes takes from 20 to 100 ms, followed by a constant flow for a time span of typically between 0.5 and 10 s.
The problem in the case of rapid draw-off processes is that the transient stabilizing phase of the magnetic field negatively affects the measuring accuracy, since during the stabilizing time intervals no measurements, or at least no accurate measurements, can be made. This is of critical significance especially at the beginning of the draw-off process where the flow rate changes quite rapidly. If the medium is drawn off during that very stabilization phase of the magnetic field, any volumetric determination will be impossible or inaccurate at best. In contrast to that, an error introduced by the transient phase of the magnetic field poses no problem in the case of a constant flow rate since interpolations can be readily applied.
It is therefore the objective of this invention to present a magnetoinductive flow measuring method that permits highly precise measurements even in high-speed draw-off processes.
The magnetoinductive flow measuring method that achieves the objective derived and specified above is characterized in that the flow metering operation is initiated the moment an accelerated flow rate is detected.
This means that the flow metering process, typically involving a switching of the magnetic field with the concomitant magnetic-field stabilization problem explained above, does not begin until the detection of an increasing flow rate signals the start of a draw-off cycle. In this context the flow may be measured by practically any prior-art flow metering technique. However, a preferred mode of implementation of this invention provides for the periodic alternating of the magnetic field during the flow measuring process with particular preference given to the use of a switched, pole-reversible constant-current field.
The flow measuring operation is not initiated for as long as a constant flow is detected. In this connection, a preferred embodiment of the invention provides for the magnetic field to be kept constant for as long as a constant flow is detected. xe2x80x9cKept constantxe2x80x9d in this case means that, in any event, the polarity of the magnetic field is not periodically alternated as it would be during the flow measuring operation. In particular, xe2x80x9ckept constantxe2x80x9d signifies that, compared to a measuring operation, the magnetic field is held in a constant state for distinctly longer periods. The magnetic field is to be kept constant along that line for as long as the flow rate detected remains constant. Of course, a constant flow rate that would keep the magnetic field constant is usually a flow rate of constantly zero.
It should be pointed out that, while during the constant state of the magnetic field, a medium flow through the measuring tube is inherently detectable by tapping the voltage differential between the measuring electrodes, that does not constitute a flow measuring operation as defined by the invention. Yet with the magnetic field in a constant state, it is possible to derive an indication, indeed a quantitative indication, of the augmentation of the magnetic field engendered by an accelerated flow rate.
The magnetoinductive flow measuring method is preferably further enhanced by the capability of terminating the flow measuring process as soon as the flow value detected drops below a predefined minimum level. Accordingly, in the case of a periodically reversed magnetic field, that magnetic field assumes a constant state as soon as the flow value measured drops below a predefined minimum value.
To gauge the flow rate for controlling the magnetic field in the above-described fashion, it is entirely possible to use an additional flowmeter. A preferred embodiment of the invention, however, provides for the acquisition of the flow rate for controlling the magnetic field through the same magnetoinductive flowmeter in which the magnetoinductive flow measuring method of this invention is implemented.
In addition, a preferred implementation of the invention employs a predefined sampling rate for detecting the flow volume. Sequential sampling values obtained with a constant magnetic field can thus be compared, with a change in the flow-sampling values to a point exceeding a predefined threshold triggering the periodic alternation of the magnetic field. Specifically, a preferred version of the invention provides for two consecutive flow-sampling values, obtained with a constant magnetic field, to be multiplied by +1 or xe2x88x921, to derive from these flow-sampling values multiplied by +1 or xe2x88x921 a flow-rate mean value which, when it exceeds a threshold value, triggers the periodically alternating reversal of the magnetic field. This utilizes the above-mentioned effect whereby, although in the case of a constant magnetic field no accurate flow measurement and thus no actual flow measuring operation is possible due to galvanic noise, it is definitely possible even in quantitative terms to detect a particular flow augmentation signaling the start of a draw-off process.
Where a predefined sampling rate serves to acquire the flow rate, i.e. to detect a change in the flow rate, a preferred embodiment of the invention provides for the predefined sampling rate extraneous to the flow measuring operation to be higher than the predefined sampling rate within the flow measuring operation. In more specific terms, this means that for a constant magnetic field the sampling rate will be higher than the sampling rate for a periodically alternating magnetic field. In the case of a periodically alternating magnetic field, the sampling rate is determined by the switching frequency of the magnetic field, in that for each half cycle, the magnetic field is sampled once through integration of the signal voltage following the stabilization time of the magnetic field. Where the design concept provides for the sampling rate extraneous to the flow measuring operation to be higher than that within the flow measuring operation, it is particularly desirable to make the sampling rate extraneous to the flow measuring operation a multiple integer of the sampling rate within the flow measuring operation. It is important in this connection that the sampling rate outside the measuring operation be independent of the stabilization process of the magnetic field, given that the magnetic field is constant. The higher the sampling rate outside the flow measuring operation, the more accurately the start of a draw-off process can be detected. The sampling rate is no longer limited by the frequency of the magnetic field reversal, but only by the quality of the electronics employed and especially that of the A/D converter serving to digitize the voltage measurement.